U.S. Ser. No. 08/350,913 describes a novel process for separating mixtures of finely divided solids which represents a significant departure from known froth flotation and selective flocculation processes. The process overcomes many of the deficiencies of prior art froth flotation and selective flocculation separation processes and provides means for producing novel kaolin pigment products when applied to kaolin clay.
The procedure utilizes selective flocculation of constituents in a previously dispersed aqueous mineral pulp, preferably a pulp dispersed with sodium metasilicate and sodium polyacrylate. The pulp is dispersed in the sense that the particles are not aggregated with each other. The dispersed pulp is preconditioned for subsequent selective flocculation by the addition of both a fatty acid and a water soluble source of a polyvalent metal cation. The quantities of fatty acid and polyvalent metal cation are insufficient to flocculate components in the dispersed pulp. When anionic polymer is added to the preconditioned dispersed pulp, a dense flocculated phase forms virtually instantaneously and settles rapidly as a dense, viscous, gelatinous bottom layer; the top layer is a dispersed fluid pulp containing the non-flocculated mineral particles. The flocculated phase also contains virtually all of the fatty acid and polyvalent cations introduced into the pulp. Separation of the lower dense gelatinous layer from the remainder of the pulp is readily accomplished by decantation or other conventional unit operation.
In the process the pulp is not subjected to froth flotation after introduction of fatty acid and polymer; nor is froth flotation used to accomplish the separation of the lower flocculated phase from the upper dispersed phase.
In an especially preferred embodiment, the invention is practiced with impure kaolin clay containing discrete particles of at least one colored titaniferous impurity and the kaolin and the impurities are so fine that they do not respond satisfactorily to conventional froth flotation processes such as ULTRAFLOTATION or TREP. The dispersant used in purifying such kaolins in accordance with the invention is preferably sodium metasilicate supplemented with sodium polyacrylate. Examples of such ultrafine kaolins are those mined in East Georgia, U.S.A. These clays have an average particles size below 0.5 micron and are presently beneficiated by selective flocculation using a weakly anionic polymer, followed by addition of copious quantities of salt to facilitate sedimentation of the flocs and multiple washing steps.
The invention described in U.S. Ser. No. 08/350,913 constitutes a significant breakthrough in the beneficiation of a host of very finely divided mineral mixtures that can provide a significant economic benefit over presently practiced technologies. For example, high brightness kaolin products (90% GE brightness and higher) can be produced without froth flotation. In some cases the high brightness kaolin products can be produced without conventional post processing procedures intended to increase brightness, such as, for example, bleaching and magnetic separation. This is explained by the fact that the process can achieve such a significant reduction in the quantity of colored impurities that conventional downstream beneficiation operations may not be needed to produce kaolin products of desired brightness. In some cases, preliminary degritting (necessary in most kaolin beneficiation schemes) can be omitted because the grit can be removed in the settled flocculated impurity layer. The process does not introduce the undesirable soluble salts introduced during prior art selective flocculation processes. This can provide significant cost reduction in kaolin processing because multiple washing steps are not needed. In fact, the multivalent metal cations present in the kaolin crude or introduced during processing may be picked up substantially quantitatively in the flocculated layer, thus not impairing the rheology of the purified kaolin. Beneficiated kaolin products having remarkably good rheology can be produced.
There is a long history of efforts devoted to the production of low brightness pigments from waste streams obtained by purifying kaolin clay by means such as magnetic purification or froth flotation. Nonlimiting examples of patents relating to this aspect of clay beneficiation include:
U.S. Pat. No. 5,190,615 Low Brightness Functional Pigment from Process By-Product Kunkle, et al.
U.S. Pat. No. 4,014,709 Opacifying Pigments and Methods for Making Same Dykstra, et al.
U.S. Pat. No. 5,047,375 Method for Producing High Opacifying Kaolin Pigment Dunaway, et al.
U.S. Pat. No. 5,154,767 Low Brightness Functional Pigment from Process By-Product Kunkle, et al.
The major problem with by products from either magnet rejects or froth flotation wastes is the high viscosity (poor fluidity) of the materials. Fluidity is defined herein as % solids where the Brookfield viscosity is 500 cps or below @20 rpm using No. 2 spindle. For example, pigment products that form slurries having a maximum of 65% solids have poorer fluidity than products that can form 70% solids slurries having such a viscosity measurement. In the case of magnet rejects, the solids at which the rejects are recovered are very low, typically in the range of 3-10% by weight. It is not economical to process such dilute streams unless costly chemicals are added for dewatering purposes.